This invention relates to the treatment of wells by positioning cement slurries therein. More particularly this invention relates to the use of a lightweight thixotropic cement slurry having zero water separation to cement wells adjacent to or above gas-bearing formations to avoid problems resulting from gas channeling through the cement slurry.
Oil well cementing practices have been used in completing wells drilled into the earth since at least the early-1900's and oil well cements have been standardized by the American Petroleum Institute (API). In this standardization, the term "oil well cement" refers to Portland cement.
Common oil well cementing practices include placing cement plugs in a wellbore to kill the well or block the flow of fluids up the wellbore; squeeze-cementing to force cement behind casing and into earth formations for such remedial purposes as sealing troublesome fluid passageways, filling voids and vugs in the formation, and combatting lost circulation problems; and cementing pipe or casing in a well.
In the completion of a well drilled into the earth, pipe or casing is normally lowered into the well and a cement slurry is pumped down the casing and up the annular space formed between the casing and the wall of the well. The cement slurry is then maintained in the annular space to allow it to set and bond the pipe to the well wall and thereby hold the pipe in place and prevent fluids from flowing behind the pipe. Various additives and formulations have been used to provide slurries having particularly desirable properties. For example, clays selected from the group of bentonite and attapulgite have been added to cement slurries as have sodium chloride, calcium chloride, dispersing agents, and gypsum.
Problems have occurred in gas cutting or channeling through the cement slurries placed in wells. In cementing casing in a well gas may flow behind the casing and through the cement slurry placed there to the surface of the earth, or may flow into lower pressured formations which communicate with the well where such gas is usually lost. The term "flow-after-cementing" has been used to characterize this phenomenon.
In an article entitled "Gas Leakage in Primary Cementing-A Field Study and Laboratory Investigation" by W. W. Christian, J. Chatterji, and G. W. Ostroot, published in the Journal of Petroleum Technology, November 1976, pp. 1361-1369, such problems and a proposed solution thereof are discussed. It is there said that with deeper well completions across gas producing horizons, especially liner cementing completions, the problems of gas leakage have become a major concern. In such completions gas leakage poses substantial problems not only in the form of potential blowouts, but also in the loss of already scarce natural resources. It is there said that recommended practices for minimizing gas leakage may be classified in two categories. The first concerns methods to obtain better bonding of the cement to both pipe and formation surfaces, and the second concerns methods that prevent entry of gas into the cemented column. This article describes an attack on the problem by the route of using fluid-loss additives to prevent entry of gas into the cemented column. Fluid-loss additives have previously been used in cement slurries. The fluid-loss consideration in primary and squeeze-cementing practices have been based on standard API fluid-loss tests. These tests use as a filter medium a 325-mesh screen at varying differential pressure. No direct method has been proposed to define what constitutes a good or adequate level of fluid-loss control. This article discusses, beginning at page 1366, a laboratory-test development of a new high-temperature fluid-loss additive. It is there said that earlier discussions have fairly well established the importance of fluid-loss additives in preventing gas migration. It was realized however that to reduce theory to practice a fluid-loss control additive was needed that could withstand high-circulating temperature in a salt medium and give a fluid-loss of about 50 ml under API fluid-loss test conditions at 190.degree. F. A polymer was developed with proper molecular weight and molecular configuration that was found to be thermally stable in high salt concentrations. Field results using this polymer are discussed.
In British Pat. No. 1,460,508 there is described another process for cementing a well which penetrates a gas-containing geological formation to minimize gas diffusion through the cement or along the contact surface between the well casing and the wall of the well. The process comprises injecting between the well casing and the wall of the well a settable cement slurry which contains a foaming agent in such an amount as to enable the slurry to form a foam with gas of the formation, and allow the cement to set and form a substantially gas-tight cement layer between the well casing and the wall of the well.
In U.S. Pat. No. 3,197,317 to Freeman D. Patchen there is described the use of low-density cement slurries in wells, which cement slurries are formed by the addition of attapulgite to Portland cement slurries. The Portland cement slurries may contain other materials, including calcium chloride, sodium chloride, and filter-loss additives. Sea water may be used in forming the cement slurry.
In U.S. Pat. No. 3,581,825 to Joseph U. Messenger there is disclosed a method of treating wells drilled into the earth, which method is particularly applicable for cementing behind casing located in wells drilled through permafrost zones. Slurries of calcium aluminate cement, clay selected from the group of bentonite, attapulgite, and mixtures thereof, and water are formed and placed in the wells to be treated.
In an article entitled "Specialty Cements Can Solve Special Problems", by Pat N. Parker, The Oil and Gas Journal, Feb. 28, 1977, pp. 128-131, there is discussed lightweight cements that are made from Portland cement and calcined shale. These cements have no API specifications but have been widely accepted. Two such low-density cements are sold under brand names of "Trinity Lite-Wate" and "TXI Light Weight". A typical analysis of TXI Light Weight cement, which is a highly sulfate-resistant cement, is there given as follows: SiO.sub.2 -- 37.2%; Al.sub.2 O.sub.3 -- 14.8%; Fe.sub.2 O.sub.3 -- 5.5%; CaO -- 34.1%; MgO -- 0.9%; and SO.sub.3 -- 4.9%. A typical analysis of Trinity Lite-Wate cement, which is a highly sulfate-resistant cement with low permeability is also there given as follows: SiO.sub.2 -- 38.3%; Al.sub.2 O.sub.3 -- 13.0%; Fe.sub.2 O.sub.3 -- 5.2%; CaO -- 35.7%; MgO -- 1.6%; and SO.sub.3 -- 2.4%.